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Didn't used to happen so makes me think it is wear.
Compact chainset, when in smaller cogs on the rear and shifting to 50t the chain on occasions fails to engage properly and pings off onto the crank. It is not a front derailer issue as winding in the high screw means it fails to shift.
I replaced the chain, still does it. Thought it might be a worn front ring. Replaced that still does it.
Could it be something to do with tension on the rear derailer? It is worse the smaller the cog, almost always happens in the bottom 3.
Might be making it up but I also think the cassette / derailer is slightly noiser.
Rear shifting is fine so it is indexed right. Jockeys look okay. Something to do with b tension screw, never adjusted it in 7000 miles, so odd if it is that.
Bent outer front mech plate?
It looks okay. No signs of damage. If it happened every shift I'd say yes, but it's like 1 in 25 in the mid cassette down to 1 in 5 on the lowest cog. Really annoying because it kicks in often when I change up at around 17-18mph so I either risk it, or ride cross chain in either too high or low cadence.
Should be relatively cheap to replace so I'll give that a go next
Would a slightly too long chain cause slack to enable is to not engage properly? Rather than derailer tension?
Is it a braze on front mech?
I had issues with a Sram front mech recently and discovered the plate holding the front mech was loose.
Tightened up the bolts and it worked fine.
Wasn't completely obvious at first.
Swapped mech and cable before I checked!
Might be your shifter not pulling the cable fully, had something similar with a 2nd hand Dura-ace shifter bought off ebay(was spares or repairs)
Flushed it out with brake cleaner and regreased and worked fine ever since.
Force 10 speed? If so, the front mech will have gone to shite; happens after a while. Replace it with an old shimano one and it should be fine.
I ended up completely taking the derailer off, cleaning and refitting it, lower than its been for the last 7000 trouble free miles. It is better, now hesitates to shift a bit but doesn't throw the chain off which is better. No idea how it happened but it appears to have taken a knock at some point even though it looks fine. Still new one is £21 from zee Germans so not the end of the world. Shame I replaced a chain and front ring which were probably fine....
First ride to check it over in a bit.